util/disk_cache: Add marker on cache usage.
util/disk_cache: Add marker on cache usage.
To be able to delete it later if it isn't used. Checking the mtime
on e.g. the index doesn't work if we always hit the cache so add
something that writes unconditionally.
Only once per day to avoid excessive writes.
(See https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22339)